Abstract
ABSTRACT V3890 Sgr is a recurrent nova that has been seen in outburst three times so far, with the most recent eruption occurring on 2019 August 27 ut. This latest outburst was followed in detail by the Neil Gehrels Swift Observatory, from less than a day after the eruption until the nova entered the Sun observing constraint, with a small number of additional observations after the constraint ended. The X-ray light curve shows initial hard shock emission, followed by an early start of the supersoft source phase around day 8.5, with the soft emission ceasing by day 26. Together with the peak blackbody temperature of the supersoft spectrum being ∼100 eV, these timings suggest the white dwarf mass to be high, $\sim 1.3\, {\rm M_{\odot }}$. The UV photometric light curve decays monotonically, with the decay rate changing a number of times, approximately simultaneously with variations in the X-ray emission. The UV grism spectra show both line and continuum emission, with emission lines of N, C, Mg, and O being notable. These UV spectra are best dereddened using a Small Magellanic Cloud extinction law. Optical spectra from SMARTS show evidence of interaction between the nova ejecta and wind from the donor star, as well as the extended atmosphere of the red giant being flash-ionized by the supersoft X-ray photons. Data from NICER reveal a transient 83 s quasi-periodic oscillation, with a modulation amplitude of 5 per cent, adding to the sample of novae that show such short variabilities during their supersoft phase.
Highlights
Novae are caused by thermonuclear explosions within an interacting binary system
Absorption edges with energies fixed at 0.67, 0.74 and 0.87 keV (H-like N, Helike O and H-like O) were included, with the optical depths allowed to vary freely; Fig. 7 demonstrates how the optical depths of these three edges vary with time
V3890 Sgr is a recurrent nova seen in eruption three times between 1962 and 2019
Summary
Novae are caused by thermonuclear explosions within an interacting binary system. Those termed classical novae (CNe) have been observed in outburst only once, while a small minority, the recurrent novae (RNe), have been detected in outburst more often, likely due to their higher white dwarf (WD) masses and accretion rates (Schaefer 2010). Continued mass transfer eventually ignites nuclear burning at the base of the accreted envelope; this causes the pressure to reach a sufficient level to trigger a thermonuclear runaway (TNR; see Bode & Evans 2008; Woudt & Ribeiro 2014, for review articles). Following this explosion, the ejected material obscures the WD surface from view. Spectra were binned such that they have a minimum of 1 count bin−1 to facilitate Cash statistic (Cash 1979) fitting within xspec (strictly speaking, a modified version for the case where there is a background file4; Arnaud 1996), and the abundances from Wilms, Allen & McCray (2000), together with the photoelectric absorption cross-sections from Verner et al (1996), have been assumed when using the Tubingen-Boulder Interstellar Medium (xspec/tbabs) absorption model
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